In recent years, the information recording technique, particularly the magnetic recording technique, has remarkably advanced following the development of information technology. An aluminum substrate has been widely used as a substrate for a magnetic recording medium for an HDD (hard disk drive) or the like being one of media for such magnetic recording. However, following the reduction in size and thickness and the increase in recording density of a magnetic disk, a glass substrate excellent in substrate surface flatness and substrate strength as compared with the aluminum substrate has been increasing in demand.
Further, following the increase in density of the magnetic recording medium, a magnetic head has also shifted from a thin film head to a magnetoresistive head (MR head) and to a giant magnetoresistive head (GMR head), wherein the flying height of the magnetic head from a substrate has been decreasing. There are instances where such a magnetic head mounted with a magnetoresistive effect element causes a thermal asperity failure as its inherent failure. The thermal asperity failure is a failure in which a read error occurs due to heating of a magnetoresistive effect element caused by adiabatic compression of air or contact when a magnetic head passes a minute convex or concave shape on a surface of a magnetic disk while flying over it.
Therefore, with respect to a magnetic head mounted with a magnetoresistive effect element, the surface of a magnetic disk is required to have extremely high-level smoothness and flatness. Further, if a magnetic layer is formed on a glass substrate with dust or foreign matter adhering thereto, convex portions are formed and, thus, high-level cleaning capable of completely removing the dust or foreign matter is also required for the glass substrate.
In addition to the smoothness and flatness of the surface of the magnetic disk, strict precision management is required for dimensional error of the inner diameter of a circular hole formed in the center of the magnetic disk. This is because the dimensional error of an inner peripheral end face of the magnetic disk directly affects the installation precision when the magnetic disk is fitted on a spindle motor of an HDD. Further, when the inner diameter dimensional error is large, there arises a possibility of causing a mechanical error in the stacking servo (writing of servo information on a magnetic disk) implemented before the magnetic disk is assembled into a magnetic disk device such as an HDD or a possibility of causing a problem in fitting to a spindle during disk stacking. The inner peripheral end face of the magnetic disk is small in surface area relative to the main surface and thus when the rotating center of the magnetic disk is offset due to the inner diameter dimensional error, it is difficult to locate a head of the HDD at a correct position on the HDD and thus it becomes impossible to record/reproduce data.
Further, since reading/writing of data is performed while rotating the magnetic disk at high speed, it is necessary to prevent fluctuation of data on the magnetic disk even during that high-speed rotation. Therefore, the precision management of the inner diameter dimensional error of the substrate for the magnetic disk becomes particularly important.
Further, paying attention to data access by the HDD, in order to accurately store/reproduce data on the magnetic disk assembled into the HDD, a servo pattern serving as a positioning index is written on the magnetic disk in advance. This writing of the servo pattern is performed by fitting the magnetic disk on an apparatus called a servo writer. Then, the magnetic disk written with the servo pattern is once removed from the servo writer and then is fitted as a product on the spindle motor of the HDD.
When the inner diameter dimensional error of the magnetic disk is large, the position of the servo pattern and that of the recording/reproducing head of the HDD as a product are offset from each other when assembling the magnetic disk into the HDD and, therefore, data recording/reproduction cannot be performed normally after all. A technique is disclosed that adjusts alignment for correcting such a positional relationship (e.g. Patent Document 1), but there is no radical solution to suppress the inner diameter dimensional error.
As described above, the requirement for the increase in recording density of magnetic disks has been further increasing in recent years and thus the inner diameter dimensional error management much stricter than conventional is required for glass substrates for the magnetic disks.    Patent Document 1: JP-A-2004-199841